1. Field of the Invention
The present invention relates to an electronic chronometry system involving both the time measuring process and the corresponding chronometric device. More particularly the invention relates to measuring systems exhibiting a resolution of more than 100 picoseconds.
2. Discussion of Background
Electronic chronometers for nonrepetitive phenomena, which measure the time interval between a starting pulse and a stopping pulse, often proceed by counting periods of a clock having a well known frequency. Generally, this time base circuit is constructed with a temperature-compensated high-stability quartz oscillator. Time T to be measured is then equal to N.tau. to within .+-..tau./2, .tau. being the clock period, N being the number present in the counter which is triggered by the starting pulse and stopped by the stopping pulse.
When a measurement resolution on the order of hundreds of picoseconds or less is desired, the time resolution of electronic counters is not sufficient, and generally the amplitude-time conversion technique is used. The starting pulse causes the starting of a sawtooth or ramp which is expressed by a voltage in the form V=kT where k is a constant. The stopping pulse causes a blocking of this linear variation.
Quantification of time can be done in several possible ways. One of the most used is the multiplication of time t by a factor K, time Kt being measured by the clock counting method already mentioned.
To obtain this multiplication factor, the ramp is achieved by the charging of a capacitance C by a constant current I (V=It/C, the terminal voltage of the capacitance). The latter is then discharged by a current that is also constant and of well determined value i given by i=I/(K-1) which give an overall time Kt for the charging plus discharging. Thus, there is produced an expansion by K of the charging time for the measurement, the resolution then being equal to .tau./K.
The relative precision of the ramp chronometer is, on many occasions, inferior to that of counting chronometers. Also, when long times are to be measured with a quantification on the order of some hundreds of picoseconds or less, association of a clock period counting, called main counting, and of ramp verniers are used. This technique is described particularly in the article by Ronald Nutt titled "Digital Time Intervalometer," in The Review of Scientific Instruments, vol 39, No 9, September 1968, pp 1342-1345. This process operates as follows:
The starting pulse causes the start of a ramp-shaped voltage V(t) which is stopped, at the end of time T1, by the first following clock pulse.
Since the phase between the starting pulse and the clock a priori has some value, time T1 will be between 0 and .tau..
Voltage V (T1) is then converted into the form of an expanded time as indicated above and is digitized (time expansion and analog-to-digital conversion).
The stopping pulse in turn causes starting of a ramp and, like the starting pulse, it is stopped by the first clock pulse that follows the end of a time T2. The stopping pulse blocks the main counter only after taking this same clock pulse into account. The counter indicates N. The time measured is then given by: EQU T=N.tau.+(T1*-T2*)
.tau. being the clock period, T1* and T2* then being the quantified values of T1 and T2. In the case of the ramp vernier with time expansion and use of the main clock, the quantum is equal to .tau./K.
To avoid uncertainties resulting from chance coincidences of starting and stopping instances with the clock, and to avoid corresponding cases of ambiguity, the ramp stops are produced on the second following pulse (or on the second front of given direction, called the active front, by a clock signal formed by pulses of a certain width. The verniers thus work in a time field between .tau. and 2.tau.. The measurement principle remains unchanged.
When it is desired to obtain very fine time resolutions, the linearity character of the sawtooth, and of the associated digitizing, takes on great importance and it is extremely difficult to go below about a hundred picoseconds.
An aim of the invention is to escape these limitations by using a process that makes it possible to compensate for the deficiencies resulting from nonlinearity and, by so doing, to correct the measurement so that the resolution achieved is less than 50 picoseconds.